1. Field of the Invention
The present invention relates to a differential unit which can execute a differential limit and a differential lock.
2. Description of the Related Art
In a conventional differential unit there is a structure which controls to fasten a multiple disc friction clutch on the basis of an electromagnetic force so as to limit a differential rotation of the differential unit. In this differential unit it is possible to obtain an appropriate handling property in correspondence to, for example, a weather condition, a road surface condition and a traveling condition in an on-road state, by controlling a differential limit force generated by the multiple disc friction clutch in correspondence to traveling conditions.
In an off-road traveling, there is a case that the differential rotation of the differential unit is locked up in accordance with a demand of an accomplishing property. At this time, in the differential unit if the fastening force of the multiple disc friction clutch is strengthened by increasing the electromagnetic force, it is physically possible to lock up the differential rotation of the differential unit.
However, it is necessary to significantly increase capacities of the electromagnet and the multiple disc friction clutch in order to obtain a predetermined lockup torque. Accordingly, there is actually a risk that an increase of weight and an increase of size are generated. Further, when the differential rotational force having an excessive load is input the multiple disc friction clutch slips, and there is a risk that an abnormal noise is generated (for example, refer to Japanese Unexamined Patent Publication No. 2000-240685 and Japanese Unexamined Patent Publication No. 2003-232432).